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124 



BULLETIN OF THE 



tial decomposition of the fractured portions of the traps and the filhiig 

 of the interspaces by the vein matter give rise to the great width which 

 the veins sometimes attain, - thirty feet. While the veins in the nar- 

 rower portions are often filled with pure vein material, in the wider 

 parts they are composed of a breccia of decomposed trap cemented by 

 vein matter. The comby and sheet structure of the veins, the class of 

 minerals enclosed, the decomposition of the associated melaphyr, — in 

 fact, all their characters, — point out that these are true fissure veins 



filled by segregation. 



In the veins the copper is found intimately mixed with the gangue, 

 or in sheets or irregular masses. The copper in the sheet form, as 

 is much of the mass copper of the veins, extends downwards, or has 

 its sides an)roximately parallel with the walls of the vein. Oftentimes 

 the sheet bifurcates, holding some of the gangue or melaphyr between 

 its parts (818). On cutting the mass copper it is not uncommon to find 

 completely enclosed in it masses of melaphyr, (piartz, calcite, or other 

 of the vein materials. The association, structure, and the relations of 

 the copper to the vein material and to the traps, all show that it was 

 deposited in the same way the vein matter and secondary materials iu 



the traps were. 



The mclaphyrs adjacent to the veins arc often impregnated, in the 

 decomposed portions, with copper, as well us the usual secondary de- 

 posits. In certain cases it has paid to take out the parts adjacent 

 to the vein, but fallacious have proved and will prove the hopes of 

 continous profitable mining based on these local deposits. Vein mines 

 associated with local deposition of copper in the adjacent traps have 

 been and are now so abundant at Lake Superior that it would be invidi- 

 ous for us to specify any one of them. In the vicinity of Portage Lid.e 

 the old lava flows themselves arc mined. No trace or sign of a true 

 fissure vein exists in most of the beds that are mined in this locality. 

 The miuin- is confined to work upon unstratified deposits, the same as 

 if some of the more cellular and deeply buried lavas of Vesuvius or 

 ■ Etna were for any reason mined. These mclaphyrs have been gi'eatly 

 acted upon by hot waters, which have decomposed them, an.l deposited 

 the copper in an irregular, " bunchy " maimer. On the Isle Uoyale lode 

 this hot-water action is very strongly markc.l, the original basalt being 

 now greatly altered, and filled with epldote and .piartz, or other min- 

 erals (377, 378, 379, 380, 381, 382, 383, 384). 



In others the action is not so strongly marked, but more evenly dis- 

 tributed throughout the bed, as for instance the Quiiu'v. All these bed 



t 



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